1. Field of the Invention
This invention relates to a circuit which controls a semiconductor laser used in a light beam scanning recording apparatus, an optical communication apparatus, or the like. This invention particularly relates to a circuit which controls a semiconductor laser so that the output power of the semiconductor laser can change quickly in response to a change in the value of a beam intensity instructing signal.
2. Description of the Prior Art
Light beam scanning recording apparatuses wherein a light beam is deflected by a light deflector, which causes it to scan a photosensitive recording material and record information on the photosensitive recording material, have heretofore been widely used. Also, optical communication apparatuses wherein optical signals are transmitted through optical fibers have been put into practice. As one of the means for producing a light beam in the light beam scanning recording apparatuses and the optical communication apparatuses, a semiconductor laser has heretofore been used. A semiconductor laser has various advantages in that it is small in size, cheap, and consumes little power, and in that the laser beam can be modulated directly by the drive current applied thereto.
In general, the circuit for operating a semiconductor laser is provided with an automatic power control circuit (hereinafter referred to as an APC circuit) which stabilizes the output power of the semiconductor laser so that the semiconductor laser produces a laser beam having an intensity accurately coinciding with the value designated by a beam intensity instructing signal. In the conventional APC circuit, the intensity of a laser beam radiated rearwardly from the semiconductor laser (i.e. a laser beam radiated in a direction reverse to the direction along which the forwardly radiated laser beam to be used for the recording of an image, or the like, is produced) is detected. Alternatively, the intensity of part of the forwardly radiated laser beam which has been separated from the main forwardly radiated laser beam by a semitransparent mirror, or the like, is detected. The drive current applied to the semiconductor laser is controlled so that the difference between the detected intensity and the intensity designated by the beam intensity instructing signal becomes zero.
However, the intensity of a laser beam radiated rearwardly from the semiconductor laser or the intensity of part of the forwardly radiated laser beam, which has been separated from the main forwardly radiated laser beam, is markedly lower than the intensity of the main laser beam which is radiated forwardly from the semiconductor laser. Therefore, the level of the feedback signal, which corresponds to the intensity detected from a laser beam radiated rearwardly from the semiconductor laser or the intensity of part of the forwardly radiated laser beam, will be too low. Since the feedback signal affects the beam intensity instructing signal, which stabilizes the output power of the semiconductor laser, and since its level is too low, it must be greatly amplified. However, when the feedback signal is amplified greatly, a large phase lag occurs in the control system for stabilizing the output power of the semiconductor laser due to, for example, the use of too many amplifiers, so that it becomes difficult to keep the band width through the feedback loop wide. Also, the speed, with which the output power of the semiconductor laser changes in response to a change in the value of the beam intensity instructing signal, drops. Additionally, in order to obtain a wide dynamic range in the output power of the semiconductor laser, it is necessary to utilize the output of the semiconductor laser which results from both comparatively small and large values of drive current. The output power of the semiconductor laser corresponding to the application of a comparatively small drive current thereto is low, and the speed with which the output power changes in response to a change in the value of the beam intensity instructing signal is low. Therefore, in cases where both comparatively small and large drive currents are applied to the semiconductor laser, a problem occurs in that the speed, with which the output power of the semiconductor laser changes in response to a change in the value of the beam intensity instructing signal, fluctuates in accordance with the level of the output power of the semiconductor laser.
If the speed with which the output power of the semiconductor laser changes in response to a change in the value of the beam intensity instructing signal is low in cases where, for example, the value of the beam intensity instructing signal changes step-wise, considerable time lags will occur before the change in the intensity of the laser beam produced by the semiconductor laser will follow the change in the value of the beam intensity instructing signal. Such time lags make it impossible to increase the operational speed of light beam scanning recording apparatuses, optical communication apparatuses, or the like.
Various means have been proposed in order to solve the problem which occurs because of the slow rate of change in the speed with which the output power of the semiconductor laser changes in response to a change in the value of the beam intensity instructing signal. By way of example, in U.S. Pat. No. 4,849,980, the applicant has proposed a technique wherein a beam intensity instructing signal which has passed through a lead-lag filter is used in order to stabilize the output power of a semiconductor laser. (this technique will hereinafter be referred to as conventional technique 1.) Further, in this U.S. Patent, the applicant has proposed a technique wherein a bias current is fed into a semiconductor laser. (This technique will hereinafter be referred to as conventional technique 2.)
However, conventional technique 1 has a problem in that, in cases where a wide dynamic range is required in the output power of the semiconductor laser, ringing occurs with the output power and the waveform of the output power becomes distorted when the level of the output power is high. With conventional technique 2, the speed with which the output power of the semiconductor laser changes in response to a change in the value of the beam intensity instructing signal can be kept high. However, conventional technique 2 has the drawback that the dynamic range of the output power of the semiconductor laser will become narrow depending on the level of the bias current. Accordingly, in cases where a wide dynamic range is required in the output power of the semiconductor laser, only a small bias current can be fed into the semiconductor laser, and therefore the operational speed of light beam scanning recording apparatuses, optical communication apparatuses, or the like, cannot be increased very much.